This application claims benefit of priority under 35 USC xc2xa7119 to Japanese Patent Application No. 2000-88992, filed on Mar. 28, 2000 in Japan, the entire contents of which are incorporated by reference herein.
The present invention relates to a waveguide optical device and method of fabricating the same. More specifically, the present invention relates to a waveguide optical device having a stepped ridge waveguide on the side surfaces of which steps of about 0.5 xcexcm are symmetrically formed, and capable of being easily and reliably fabricated, and to a method of fabricating the same.
Examples of an optical device having a waveguide for guiding light are a light-emitting device such as a semiconductor laser, an optical modulator, and a light-detecting device (receiver) such as a waveguide photodiode. As this waveguide, a structure called xe2x80x9cridgexe2x80x9d is known. In the case of a semiconductor laser having a double-hetero structure including a cladding layer/core layer/cladding layer, for example, this laser has a stripe waveguide so fabricated that the cladding layer above the active layer has a convex section. In a waveguide of this type, the stripe portion including the active layer below the ridge formed in the cladding layer functions as a waveguide to guide light.
Although such a waveguide is sometimes called xe2x80x9cribxe2x80x9d or xe2x80x9cstrip loadxe2x80x9d, these waveguides are generally called xe2x80x9cridge waveguidesxe2x80x9d in this specification.
FIG. 9 is a perspective view showing a typical structure of a ridge waveguide semiconductor laser relevant to the present invention. That is, this laser shown in FIG. 9 is an InGaAsP/InP-based semiconductor laser used in the field of long-distance, high-speed optical communications. The construction of this laser will be described below following the fabrication procedure.
First, an n-InP lower cladding layer 2, an InGaAsP waveguide core layer/active layer 3 (about 0.1 xcexcm thick) having an MQW (multiple-quantum well) structure, a p-InP first upper cladding layer 4 (about 0.15 xcexcm thick), a p-InGaAsP etching stop layer 5 (about 0.05 xcexcm thick), a p-InP second upper cladding layer 6 (about 1.3 xcexcm thick), a p-InGaAsP barrier buffer layer 9 (about 0.04 xcexcm thick), and a p+-InGaAs contact layer 10 (about 0.1 xcexcm thick) are formed flat by crystal growth on an n-type (100) InP substrate 1. The barrier buffer layer 9 is formed to buffer the rectification properties by the barrier between the p+-InGaAs contact layer 10 and the p-InP second upper cladding layer 6. This barrier buffer layer 9 has a bandgap corresponding to a wavelength of 1.3 xcexcm which is an intermediate composition between these layers 6 and 10.
Subsequently, a sulfuric acid-based etchant (e.g., 4 sulfuric acid+1 hydrogen peroxide+1 water) is used to etch away the p-InGaAsP barrier buffer layer 9 and the p+-InGaAs contact layer 10 so as to leave a stripe portion about 2 xcexcm wide behind.
These layers are used as masks to perform etching by using a hydrochloric acid (HCl)-based etchant. Consequently, the p-InP second upper cladding layer 6 is substantially vertically etched down to the p-InGaAsP etching stop layer 5. Since the HCl-based etchant acts only on InP, this etching accurately stops at the etching stop layer 5. Accordingly, a ridge waveguide having a convex section can be formed.
One modification of the ridge waveguide as shown in FIG. 9 is a so-called xe2x80x9cburied waveguide constructionxe2x80x9d in which a waveguide layer is formed into a stripe and a medium having a low refractive index is buried around the stripe. This buried waveguide structure can also guide light by a refractive index difference in the lateral direction of the ridge.
In an optical device using the ridge waveguide shown in FIG. 9, however, it is difficult to simultaneously control the junction capacitance and transverse mode stability of the ridge.
That is, when an optical device including this waveguide is used as a semiconductor laser or as an electro-absorption type optical modulator integrated monolitically with the laser, electrodes 20 and 21 must be formed on the upper and lower surfaces of the waveguide. Unfortunately, an electric current supplied to the device via these electrodes spreads in the lateral direction in the first upper cladding layer 4 in the lower portion of the ridge stripe. This results in insufficient current focus to the core layer/active layer 3 of the waveguide. As a result, a non-active current; increases, and the threshold current of the semiconductor laser increases.
Also, the junction capacitance increases because the active layer 3 exists in the lateral direction. When the junction capacitance increases, the device cannot be easily modulated at high speed any longer either as a laser or as a modulator. In particular, modulation at 10 Gbps (Gigabits per second) or more becomes difficult.
To avoid these problems, the thickness of the first upper cladding layer 4 can be decreased to zero. When this is the case, however, light spread in the lateral direction of the ridge becomes insufficient, and this makes the transverse mode of the waveguide unstable.
Another option is a xe2x80x9cstepped ridge waveguidexe2x80x9d in which a ridge is formed into the shape of a staircase.
FIG. 10 is a perspective view showing an outline of the structure of this xe2x80x9cstepped ridge waveguidexe2x80x9d. In FIG. 10, the same reference numerals as in FIG. 9 denote the same parts explained above in connection with FIG. 9, and a detailed description thereof will be omitted.
When the first cladding layer 4 is slightly wider than and projects in the form of steps from the second cladding layer 6 as shown in FIG. 10, a margin for spread in the transverse mode is formed, so the transverse mode stabilizes. Since the first cladding layer 4 is patterned, the spread of a current in the lateral direction can also be suppressed.
It is, however, extremely difficult to symmetrically form the steps as shown in FIG. 10 with high controllability by a projection width of about 0.5 xcexcm. The reason is that if common photolithography is used, it is difficult to symmetrically align a mask pattern with a ridge mesa in which steps are once formed.
The present invention has been made in consideration of the above problems, and has as its object to provide a waveguide optical device and a method of fabricating the same, which can easily implement a stepped ridge waveguide structure.
To achieve the above object, a waveguide optical device of the present invention is a waveguide optical device comprising a waveguide for guiding light, characterized in that the waveguide is a ridge waveguide in which a core layer is vertically sandwiched between cladding layers having a refractive index lower than that of the core layer, and one of upper cladding layers is formed into the shape of a mesa stripe, the upper cladding layers comprise at least a first cladding layer formed on the core layer, a first thin-film layer formed on the first cladding layer, a second thin-film layer formed on the first thin-film layer, and a second cladding layer formed on the second thin-film layer, steps are formed on the side surfaces of the mesa stripe such that the width of the mesa stripe increases in a lower portion close to the core layer, and the first thin-film layer is exposed to the steps.
The first and second cladding layers and the first and second thin-film layers are made of materials by which the rates of side etching from the side surfaces of the mesa stripe satisfy (second thin-film layer) greater than (first thin-film layer) greater than (first and second cladding layers) with respect to a first etchant, and satisfy (first and second cladding layers) greater than (first and second thin-film layers) with respect to a second etchant different from the first etchant. Accordingly, steps corresponding to a difference between the side etching amounts can be reliably formed.
A third cladding layer can be inserted between the first and second thin-film layers.
The first and second thin-film layers can also be stacked adjacent to each other. This further simplifies the device configuration and fabrication steps.
More specifically, the first and second cladding layers can be made of InP, and the first and second thin-film layers can be made of an InGaAlAsP-based compound semiconductor.
In this specification, an xe2x80x9cInGaAlAsP-based compound semiconductorxe2x80x9d includes all compositions within the ranges of 0xe2x89xa6xxe2x89xa61, 0xe2x89xa6yxe2x89xa61, 0xe2x89xa6zxe2x89xa61, and 0xe2x89xa6(x+y)xe2x89xa61 in a composition InxGayAl1xe2x88x92xxe2x88x92yAszP1xe2x88x92z.
A method of fabricating a waveguide optical device of the present invention is a method of fabricating a waveguide optical device having a ridge waveguide in which a core layer is vertically sandwiched between lower and upper cladding layers having a refractive index lower than that of the core layer, and one of the upper cladding layers is formed into the shape of a mesa stripe, characterized by comprising the steps of forming a core layer on a lower cladding layer, forming, as the upper cladding layers, a first cladding layer, first thin-film layer, second thin-film layer, and second cladding layer in this order on the core layer, patterning the upper cladding layers into the shape of a stripe, side-etching the first and second thin-film layers exposed to the side surfaces of the stripe, by using a first etchant by which the etching rate for the second thin-film layer is higher than that for the first thin-film layer, and etching the first and second cladding layers by using a second etchant by which the etching rate for the first and second cladding layers is higher than that for the first and second thin-film layers, thereby forming steps corresponding to a difference between the side etching amounts of the first and second thin-film layers, on the side surfaces of the mesa stripe.
In this method, fine steps of about 0.5 xcexcm can be easily and reliably formed symmetrically on the right- and left-hand sides of the mesa stripe by properly selecting the materials of the first and second thin-film layers, the first etchant, and the etching conditions.
In the step of forming the upper cladding layers, a third cladding layer can be formed between the first and second thin-film layers.
The first and second cladding layers are made of InP, the first and second thin-film layers are made of an InGaAlAsP-based compound semiconductor, the first etchant is a sulfuric acid-based etchant, and the second etchant is a hydrochloric acid-based etchant. When this is the case, an InP-based waveguide optical device can be easily and reliably formed.